Many wells are drilled vertically, such as the wells that are drilled in central Texas into a producing region known as the Austin chalk. The Austin chalk formation is a tight formation. Not only is the producing zone tight, it is relatively narrow, and is generally horizontal strata about 10 or 20 feet thick.
This narrow strata is penetrated by a well which is typically drilled through the strata and therebelow, and subsequently perforations in a casing string are formed opposite the producing formation. This causes production fluids to flow from the formation into the producing well.
Eventually, the rather thin, tightly defined formation will become depleted in that region. Ordinarily, that means that the well must be plugged and abandoned because it is no longer economically useful. As the well becomes depleted, one of the choices that might extend the life is a secondary completion procedure. Rather that simply abandon the well, the well is redrilled in a very special fashion. By backing up the well can be re-entered, and a window is then milled in the casing. The window cuts out about 100-300 feet of the casing,. Through the use of a bent sub and a drill motor with a drill bit attached, the well is then continued by causing the drill bit to curve through the window cut in the casing and to drill the side. The radius of curvature is controlled. Therefore the well which is typically vertical in the first instance will then be deviated.
The deviation can be controlled to the extent that the well then is directed horizontally. If the correct point of deviation is selected and if the curvature can be controlled properly, the horizontal well can then be located along the central portion of the strata which produces the desired petroleum products. Indeed, if the strata is at some canted angle with respect to the vertical well, the redrilled well can be directed in the middle of the strata for several hundred feet. It is possible to drill laterally from the vertical well borehole upwards of 1000 feet or so.
In horizontal drilling, the drill bit is advanced into the various formations encountered by the vertical well by grinding at the face of the advancing well borehole, forming chips or cuttings which are carried to the surface in the annular space on the exterior of the drill string. This carries the chips or cuttings to the surface where they are retrieved from the drilling fluid so that the drilling fluid is recycled to be used time and again. There is a substantial downward flow of drilling fluid directed to the drill string and hence there is a substantial return annular flow around the drill string. Even when the well is deviated significantly from the vertical, the foregoing remains true.
To be sure, it is somewhat idealized to represent the drill pipe as being centered within the annular hole. Sometimes, the drill pipe will contact the sidewall and will form what is known as a keyseat. Even in that instance, however, in vertical drilling there still remains a very substantial flow space which is normally concentric around the drill pipe but which may be distorted by keyseating.
As the well is deviated intentionally from a vertical hole toward a nearly horizontal hole, and particularly when drilling with a drill motor, a different type drill bit is used and a different source of power is applied. In that instance, the drill bit is normally rotated by a drill motor which is attached at a lower end of the drill string. Through the use of a bent sub, deviated drilling is controlled.
Furthermore, gravity causes the drill string behind the drill bit to lay against the bottom of the horizontal hole. This assures in all points in time that the return fluid flow including the chips or cuttings from the drilling process must flow above or around the drill pipe but not on all sides of it. So to speak, the drill pipe settles to the bottom of the hole. This creates a larger chance of sticking which is somewhat similar to keyseating with a vertical well. The chance of sticking is increased in proportion to the length of horizontal hole. For instance, if the horizontal portion of the well is only 200 feet, there is less chance of sticking than there would be if the horizontal portion of the hole were 1000 feet.
Generally, horizontal drilling is highly desirable, but is limited in part by the tendency to stick as the horizontal hole becomes longer and longer. Indeed, a horizontal well can substantially enhance production from a formation which is otherwise depleted. By contrast, this high production rate is achieved only when the horizontal hole is fairly long. As the horizontal portion is made longer sticking becomes a greater problem.
One aspect of sticking derives from embedding of the cuttings which are dislodged by operation of the motor and drill bit. The cuttings are flushed back towards the well head and forced to flow horizontally in the hole. The flow rate of the drilling fluid is normally sufficient to carry the cuttings. There is, however, some tendency of the cuttings to settle in this flowing drilling fluid and they tend to fall towards the bottom of the horizontal hole.
The bottom side of the horizontal hole is immediately adjacent to the curving drill string. If the drill string is resting against the bottom and the chips or cuttings fall toward the bottom there is the risk that the chips will be forced under the drill pipe and become embedded in the mudcake, perhaps embedding into the formation being drilled. If this occurs, an excessive number of chips or cuttings can be collected under the drill string and thereby create something of a chip buildup or accumulation on the bottom side of the well borehole.
Thus, a need exists for a composition and method for decreasing the buildup of cuttings in the annulus of a wellbore.